DD144955A5 - METHOD FOR CLEANING THE CHANGES OF HEAT EXCHANGERS - Google Patents

Description

A j J 407 _1- Berlin, 22.11.1979

& ι 4 "vr - _{Ap p 28 G / 214 407}

55 728/12

Method for cleaning the walls of hot breakers Field of application of the invention

The invention relates to a novel process for cleaning heat exchangers, the walls of which are covered with incrustations, in particular with titanium-containing preparations, which deposit in the digestion of ores and result in a reduction of the heat exchange potential.

Characteristic of the known technical solutions

The skilled person has long been aware of numerous, often insurmountable difficulties that occur when the essential properties of heat exchangers in reaction vessels are to be maintained because a parasitic phase often occurs on the ends of these heat exchangers. Thus, in the case of digestion of ores during the digestion process, crusts, which are often very difficult to remove, form on the surfaces where the heat exchange takes place, and these crusts change the ITutzquerschnitt the digestion reactor, but especially the heat exchange coefficient of the heat exchanger itself.

Since these crusting cause a very detrimental development of the basic properties and characteristics of the heat exchanger and digestion reactors are already

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Numerous more or less happy attempts have been made to. To clean heat exchangers and digestion reactors and to remove the deposited crusts.

One of the most advanced classical solutions known and described in the literature for combating this process is a first type of cleaning method in which the walls to be treated are mechanically encrusted by vigorous scraping, impacting or impacting Shocks, vibrations, by brushing, sandblasting, etc., or even by a combined action of such agents.

Another type of cleaning method is that the hands, the ^xi tion vessel treated chemically and the deposited on these crusts are dissolved or decomposed. One has, for example, attempts to purify with crusts which form on supervisors chlußtemp erat ur s above 180 ⁰ G, covered walls characterized in that is circulated at a relatively high temperature, an acidic solution of hydrochloric acid and hydrofluoric acid in the reaction vessel.

Although such a process provides a substantial improvement over the previously known procedure, it has been shown that it can not be universally applied to all types of reaction vessels. In fact, it has been shown in experiments that the chemical action of the acid pair alone is by no means sufficient for complete removal of the crusts. It has therefore proved necessary to have the chemical action of this acid pair with a mechanical action by injecting water

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to combine under high pressure. "This method thus proves to be a combination of two remedies, chemical and mechanical; it is applied according to the principle of piston flow and is therefore useful only for tubular reactors.

Object of the invention

The aim of the invention is to provide an improved purification process for heat exchangers.

Explanation of the invention

The invention has for its object to find suitable solvents which dissolve the deposited on the walls of heat exchangers or reaction vessels titanium-containing crusts or in the course of Erzaufschlusses formed crusts of aluminum silicates, which cause a significant reduction of the heat exchange potential, and the basic properties of these walls restore.

According to the invention, the crusts are removed with an aqueous solution of hexafluorosilicic acid (hydrofluoric acid) and hydrofluoric acid; this mixture contains 3 to 30 parts by weight of hydrobromofluoric acid and the highest 10% by weight of hydrofluoric acid.

In this case, first an aqueous treatment solution is introduced, which contains only hydrofluoric acid, and then added continuously during the course of the treatment, an aqueous-hydrofluoric acid to continuously

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to regenerate the hydrofluoric acid,

As already stated, after a certain period of operation, crusts on the heat-transferring surfaces of the devices for the decomposition of ores are deposited which are essentially titanium-containing, solid and compact and which are formed during the digestion.

According to the invention, it has been attempted to remove these particularly disturbing deposits, for example by means of an aqueous solution of particularly active acids, at reasonable digestion concentrations and temperatures.

First, an aqueous hydrofluoric acid was used »It was found, for example, that at a reasonable digestion temperature of z. B. 60 ⁰ C at an HP concentration of 4 Gew.-y $ not more than 5 % of the deposits are solved.

In the same way when using an aqueous hydrofluoric acid (H ₂ SiPg) was found that at a concentration of 13 wt .-? S HpSiEV and at the same digestion temperature (as above) not more than 30 % of the existing crusts could be solved.

It has surprisingly been found that with an aqueous solution, which together more than 10% hydrofluoric acid and 3 to 30 percent by _e in the mixture - containing% fluosilicic acid, due to their SYNERGISTIC EFFECT 80 to 100%

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the thus treated crust deposits can be safely removed at treatment temperatures of 20 to 80 ⁰ C.

The Applicant has been able to demonstrate that the active agent or agent for dissolving the crusts is hydrofluoric acid, but in the course of its action passivates the attacked surface of the crusts by depositing silica, and that the hydrofluoric acid reactivates the dissolution reaction by regenerating hydrofluoric acid becomes.

Preferably, the aqueous solution should contain 5 to 15 % of hydrofluoric acid and 1 to A% of hydrofluoric acid.

According to a variant of the method, which makes it possible to shorten the time required to remove the crusts in technical equipment, can be used to dissolve the separated crusts an aqueous solution of only hydrofluoric acid and this continuously the only necessary for the regeneration of Kieselfluorwasserstoffsäure IFluorws-sserst off acid are added to the extent that the crusts are dissolved, so that the concentration of free hydrofluoric acid in the treatment solution for the deposits is as low as possible and preferably 0.

In this case, the aqueous 3 _e treatment solution containing the hydrofluoric acid, hydrogen fluoride. acid are added in highly concentrated form, and

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although in a concentration Ms to 40 wt .- ^ HP in aqueous solution.

In practice, it has been found to be desirable to add the hydrofluoric acid in the form of a concentrated aqueous solution which allows regeneration of hydrofluoric acid without appreciably increasing the volume of treatment solution in the plant being decrusted.

Since the dissolution of the crusts takes place uniformly, the aqueous hydrogen fluoride solution is introduced into the technical system which is being purified in controlled quantities by means of any known device adapted for this purpose, for example by means of a metering pump.

The feed rate of the aqueous hydrogen fluoride solution is controlled so that the concentration of hydrofluoric acid in the treatment solution remains constant and practically equal to the concentration at the beginning of the treatment.

Thus, during the entire process of dissolving the crusts, the concentration of SiFg ions remains constant, while only the continuously added hydrofluoric acid is consumed continuously regenerating diatomic hydrofluoric acid. Thus, several operations of dissolving the deposited crusts can be performed with the same cleaning solution be performed, each time the solution is used, which comes from a previous cleaning process.und their concentration

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has been readjusted by the addition of hydrogen fluoride.

Ausführunsjsbeispiel

The invention will be explained in more detail below on some embodiments.

The following examples clearly show the synergistic effect of the acid pair HF and HpSiPg in aqueous solution when this acid pair is used to dissolve crust-like deposits.

Example 1 illustrates the effect of hydrogen fluoride alone,

Example 2 illustrates the effect of hydrofluoric acid ·

Example 3 shows the synergistic effect of the acid pair HP and H ₀ SiF;

Example 4 confirms the synergistic effect of the acid pair at other concentrations.

Example 5 shows the influence of the temperature on the kinetic A-scale of the dissolution of the crusts.

Example β relates to the purification of a large scale plant which is heavily encrusted with an aqueous solution containing the acid pair HF and H ₀ SiFg.

The same applies to Example 7.. ·.

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Example 8 refers to the solubilization of a heavily encrusted large scale plant with an aqueous treating solution containing hydrofluoric acid and to which is continuously added an aqueous hydrofluoric acid which ensures the continuous regeneration of the fumed silica hydrofluoric acid.

example 1

In a . · Technical test facility treated 50 kg of crusts that were incurred during the mechanical cleaning of an industrial plant. The crusts had the following composition:

₂ 42.0 G _eWo - #

CaO 22.8 wt .- #

Fe ₂ O ₃ '9.0% by weight

Al ₂ O ₃ '12.8% by weight

SiO ₂ 1.7 % by weight

Na ₂ O 3.9 wt .- ^ HpO bound

+ Miscellaneous' 7.8 wt .- $

average layer thickness 4mm.

3 1.5M hydrofluoric acid containing wt % HF was added. The -Temperatur'wurde 7 hours at 60 ⁰ C and the mixture kept stirred IJach end of this time were 1.16 kg TiO ₂ in.Lösung gone; this corresponded to a digestion yield of 5.5 % of the crust layer

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remained virtually unchanged (not open-minded). Example 2

There were treated 50 kg crusts of the same origin as in Example 1, in the same pilot plant and while maintaining the same conditions in terms of time and temperature, with 1.5 m of an aqueous Kieselfluorwasserstoffsäure, concentration 13.1

After completion of the digestion time 6.3 kg TiOp had gone into solution, corresponding to a yield or a proportion of 30 5S.

The appearance of the crusts had changed: a white precipitate had formed on the surface, which proved to be silica in the analysis.

Example 3

There were treated 50 kg crusts of the same origin as in Example 1, in the same pilot plant and maintaining the same conditions in terms of time and temperature, with 1.5 m of an aqueous solution containing 1.94 wt .-% HP and 6 , 52 wt .-% H ₃ SiP ₆ .

After completion of digestion 17.0 kg of TiOp had gone into solution, corresponding to a yield or a proportion of 81 %, the layer thickness of the remaining crusts was on average less than 1 mm.

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The aqueous solution containing the mixture on HP and HpSiPg thus showed a synergistic ability in the stone-like crusts compared _{to the} effect of HF or alone.

Example 4

There were treated 50 kg crusts of the same origin as in Example 1, in the same pilot plant and under the same conditions in terms of time and temperature. When

3 ml of aqueous solution were used which contained 3.88% by weight of HP and 13.04% by weight of H ₂ SiP ₆ .

At the end of the digestion time 20.5 kg of TiO ₂ had gone into solution, corresponding to a yield or a proportion of 97.5 SS.

The remaining crust remnants were completely decomposed and had the appearance of a powder suspended in the solution.

By increasing the concentration of the digestion solution to HP and H ₂ SiP ^, the yields in the dissolution of the crusts which are to be removed can be improved.

Example 5

After observing that the mixture of HP and H ₂ SiPg unfolded a synergistic ability to dissolve the substantially titanium-containing crusts, the influence of temperature on reaction kinetics was examined.

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For this purpose, 45 kg crusts were treated in a pilot plant, with an aqueous solution containing a mixture of 1.94 wt .-% HP and 6.52 wt .-% H ₃ SiP ₆ .

The crusts that were to be digested had the following composition:

I ¹ JLO ₂ 28.1% by weight

CaO 16.1 ^G ew .- #

Pe ₂ O ₃ 11.7 ^G ew .-%

Al ₂ O ₃ 18.3% by weight

SiO ₂ 8.7 parts by weight

O 8.2% by weight

bound

+ Verse 8,9

the volume of the digestion solution was 1.4 m.

The investigations were carried out at three temperatures; 25 ⁰ C, 00 ⁰ C and 80 ⁰ C.

Samples were taken during the digestion to determine the yield of the reaction.

All results are summarized in the following table; the reaction yield is represented by the amount in wt .-% of which has gone into solution.

4g, j ^, "si% # f -12- AP L L E 1 25 60 i 22:11 .1979 / 12 55 no sampling no sampling P 28G / 214 407 On the chi ujibemperatur ⁰ G - 6.2 % 48.0 % 728 TAB E 11.6 % 60.0 % no sampling no sampling Time 26.9 % 73.1 % 80 O Of t- / O H. no sampling 84.7 % 67, O cf C./O 0.5 no sampling End of the experiment 80 I / Ό 1 51.6 % 91 1 C * I / O 2 77.2 % 93 of the experiment 3 End of the experiment The End 4 6 7 8th 24

The table shows that the reaction rate increases as the temperature increases.

example 6

It was a large-scale autoclave with Hutz content of 42

m cleaned, in which a heating tube bundle was arranged

ρ with a heating surface of 240 m.

The height of the autoclave was 10 m, its diameter 2.5m.

The heating tube bundle of steel A 42 comprised 24 tube rakes with 8 ears each.

The mass of crusts deposited on the heating tube bundle

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was estimated at 2 tons; the layer thickness varied between 5 and 10 mm.

Before the cleaning treatment, the crusts had the following composition (in% by weight):

T Α-B E L L E 2

Lower end of the upper end of the tube bundle

Loss on ignition 3,3 5,7

SiO ₂ 2.5 2.9

Al ₂ O ₃ 5.4 6.2

Pe ₂ O ₃ 15.7 16.7

P ₂ O ₅ 1.3 1.4

CaO 27.5 24.7

40.0 37.1

₂ · 3.6 3.6

MgO 0.7 1.7

In the autoclave were added 42 m treating solution containing 1.65 wt £ HP and 7.8 wt .- $ i H ₀ SiP, - contained and

«I C. O

3 kg / nr solution of a passivant known type was added.

By circulating hot water in the heating tube bundle, the temperature was set at which the reaction began; the reaction then continued exoterm.

The temperature at the beginning of the reaction was 40 ^° C., in the case of

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action end 48 ⁰ G.

The course of the digestion was followed by quantitative determination of the titanium present in each case in the treatment solution.

The results are summarized in the following table.

TABLE

Time TiO ₂ in the ^ -sufschlusslösung

h g / l

0.5 "0.2

1 »5 0,5

5.0 2.1

6.5 · 2.9

9.5 5.7

14.5 7.7

17,5 11,6

21.5 13.4

24.0 15.4

After 24 hours of treatment, 1.7 tons of crusts were practically dissolved.

The autoclave wall was very clean. Bs remained on this some fine cuticles or pilates from still adhering crusts, which did not matter quantitatively.

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No traces of corrosion were detected. Example 7

The exchangers of a pipe system with internal diameter MT, 7 mm were cleaned.

For this purpose, the treatment solution was prepared in a stirrer-equipped reservoir; it contained 2.14 w / o HF and 5, so wt% H ₂ SiF ^. In addition, 3 kg / nr of solution had been added to passivation agents of known type.

The treatment solution was then pumped into the piping to be cleaned and circulated at a rate of 1.2 m / s, passing through the stirred tank.

First, the treatment solution was circulated in a heavily encrusted (average layer thickness 5 mm) part of the pipe system corresponding to a length of 45 m, for 12 hours at a temperature of 45 C _e

Then, the circulation of the treatment solution was extended to a group of 10 series-connected pipes, which were 660 meters in length; the treatment temperature was adjusted to 61 ⁰ C by means of hot water in a heating mantle.

After 5 hours of operation, the treatment was discontinued. Of 10 treated tubes, 5 were completely clean, the other 5 were not completely clean.

The 5 not yet completely encrusted tubes were with 8 more

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Pipes in Seihe switched, which should be encrusted. The tubes thus connected were treated with the same solution after their concentration was readjusted by adding 880 kg HP. The treatment solution ran for 8 hours at a constant temperature of 55 ⁰ C.

After this time, the exchangers of the entire tube device were clean, d. H. descaled.

The course of the digestion was followed throughout the operation by quantitative determination of the present in the treatment solution at different times

The results are summarized in the following table.

TABLE 4

Time Number of tubes containing TiO ₂ in the digestion

h were cleaning solution

7 1 1.0

12 1 2.0

13 10. 4.4

14 10 5.7

15 10 6.5

16 10 7.0

17 10 7,3 19 13 11,7 21 13 12,6

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Time number of pipes, die.ge- TiO ₂ in the

h were cleaned final solution

23 13 13.2

25 End of treatment 13 13.6

Example 8

A heat exchanger pipe system was encrusted, inner diameter of the pipes 177.7 mm. The entire system consisted of 10 series-connected pipes corresponding to a length of 66Om.

About 45 m of aqueous, 6.1% strength by weight hydrobromic acid were prepared as a cleaning solution in a vessel with stirrer, with the addition of a passivating agent of known type in an amount of 3 kg / nr of solution,

The treatment solution was pumped into the pipe system, which was to be cleaned, and passed at a rate of 1.2 m / s in a closed circuit through this system including the storage container with stirrer.

The treatment temperature was adjusted to 54 ° C. by means of hot water in the heating jacket.

Fifteen minutes after the start of treatment, 500 l / h of aqueous hydrofluoric acid containing 25 μg / t of HF were added using a metering pump.

After 4 hours the treatment was stopped and fixed.

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provided that all 10 tubes were completely encrusted and perfectly clean.

At digestion rate was followed in the course of treatment by quantitative determination of the titanium present in the treatment solution; the samples were removed from the treatment solution at fixed intervals. The results are summarized in the following table.

TABLES

Time TiO ₂ in the digestion solution

h g / l

0.5 4.0

1.0 '6.0

1.5 8.6

2.0 10.3

3,0 11,4

4.0 11.7

The results show that after 3 hours of treatment the Sntkrustung was practically completed; from the third to the fourth hour, the TiOg-C content of the treatment solution increased only slightly.

For comparison, a further cleaning test was carried out with the same apparatus having the same degree of crusting.

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The "procedure remained the same except that the treatment solution was slightly different, and had before the introduction into the plant following composition: 6.1 wt .- $ j fluosilicic acid and 1.9 wt, -% hydrofluoric acid _o throughout the treatment was the Temperature maintained at 55 ⁰ C.

After 3 hours 45 minutes 718 kg of pure hydrofluoric acid in the form of an aqueous solution containing 27 % HF was added.

The treatment was stopped after 7 1/2 hours; the pipes were clean.

The encrustation rate was monitored during the course of the treatment by quantitative determination of the titanium present in the treatment solution; the samples were withdrawn from the solution at fixed times. The results are summarized in the following table.

TABLE 6

Time TiO ₂ in the digestion solution

h g / l

1.0 5.8

2.0 7.0

3.0 7.8

3.75 7.8

4.25 8.8

5.25. 10.0

6.75 11.0

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Time TiO ₂ in the digestion solution

h S / l

7.50 11.3

The comparison shows that the cleaning was completed practically after 7 hours of treatment. Between the last two samplings of TiOp- ^ü took ehalt of - ^ ehandlungslösung little to.

The results show that when using an aqueous treatment solution containing a mixture of hydrofluoric acid and hydrofluoric acid prior to introduction into the plant which is to be cleaned, a significantly longer residence time is required to reach the same result as when an aqueous Solution is used, which contains only the hydrofluoric acid prior to introduction into the apparatus which is to be cleaned. The comparison, in particular of Tables 5 and 6, clearly shows the improvement caused by the continuous addition of hydrofluoric acid.

Claims (6)

-21- 22.11.1979 AP P 28G / 214 407 55 728/12 Claim for invention 1. A method of cleaning the walls of heat exchangers or reaction vessels which are covered with substantially titanium-containing crusts, possibly also with aluminosilicate crusts, which have formed in the course of the digestion of ores and cause a considerable reduction in the heat exchange capacity, which serves to establish the basic properties of the walls v / ieder, characterized in that the crust deposits are removed with an aqueous treatment solution containing 3 to 30 weight "-% fluosilicic acid and at most 10 wt -. containing% hydrofluoric acid. 2. The cleaning method according to item 1, characterized in that first introducing an aqueous treatment solution which contains only hydrofluoric acid and then continuously added during the treatment of an aqueous hydrofluoric acid to continuously regenerate the hydrofluoric acid « Cleaning method according to item 1, characterized in that the aqueous solution preferably contains from 5 to 15% by weight of hydrobromofluoric acid and from 1 to 4% by weight of hydrofluoric acid. «« »._, _ ₂₂ _ 22.11.1979 AP P 28G / 214 407 55 728/12 4. Cleaning method according to the items 1 and 2, characterized by adding a corrosion inhibitor to the treatment solution. 5. The cleaning method according to the items 1 and 2, characterized in that the treatment at a temperature between 20 and 80 ⁰ C made v / earth can be made. 6. The cleaning method according to item 5, characterized in that the cleaning solution comes from a previous cleaning operation, after readjustment of the composition by addition of hydrofluoric acid.